I've actually always removed the old ones, but you do have to be prepared for some tidying up for the reason xerophyte points out.

If you do decide to remove them (you can always reinstall perl using the perlinstaller package on layer2.cpanel.net) make sure you run the following after wards repeatedly until it doesn't update any more:

/usr/local/cpanel/bin/checkperlmodules

On very rare occasions I've seen the tidyup affect exim in which case you can remedy that with: